1. Field of the Invention
The present invention relates to threaded fasteners, and more particularly, to a lead-in point piloting an end of the external portion of the threaded fastener into better axial alignment with the mating internal portion of the fastener so as to reduce the chance of cross-threading and/or jamming of the external and internal thread helixes.
2. Description of the Related Technology
Threaded fastener technology is basic to the construction or fabrication of most articles of manufacture such as machines, automobiles, trains, planes, boats, engines, and the like. Threaded fasteners may be bolts, screws, studs, rods, or other substantially round members having uniform, non-uniform or tapered external helical threads that are screwably engaged into internal helical threads such as nuts, bolts, holes and the like. For proper engagement of the external and internal member portions of the threaded fasteners, the longitudinal axis of the externally threaded member portion must be substantially collinear with the longitudinal axis of the mating internally threaded member portion. If collinearity is not maintained between the externally and internally threaded member portions, then cross-threading occurs.
Cross-threading generally occurs when there is a misalignment between the externally threaded member portion, typically a screw or bolt, and the internally threaded member portion, typically a nut or other threaded internal opening. Specifically, cross-threading is the result of the threads of the two member portions attempting to engage at least one half pitch out of alignment. When this situation happens, the two member portions are not substantially collinear with each other and wedging of the threads will occur as the threaded helixes are rotated against each other. If rotation continues, then threads on one or both members will be structurally damaged.
A lead-in, such as a "pilot" or "dog" point, on an end of the externally threaded member portion, see generally FIG. 1, may be used to restrict the axial mis-alignment between the external and internal member portions of the threaded fastener. The lead-in helps reduce cross-threading and/or jamming of the thread helixes, see generally FIG. 2. The axial mis-alignment is reduced by utilizing the length and/or diameter of the externally threaded member portion lead-in which slidably engages the internal member portion, thus restricting axial mis-alignment variance therebetween. Long lead-ins, i.e., greater than three thread pitches in length, are commonly used, but short lead-ins have been used, i.e., less than three thread diameters, with less than satisfactory results. The short lead-ins have tended to cause assembly difficulties, primarily due to the following two problems:
When the externally threaded member portion having a short lead-in is inserted into the internally threaded member portion in an axially mis-aligned condition, the end of the lead-in may contribute to thread jamming by becoming lodged in an internal thread impression, see generally FIG. 2. This prevents the internal and external thread helixes to feed axially relative to one another, thus preventing the surfaces of the internal and external fastener member portions from sliding into correct axial alignment. When this occurs, continuing to turn the fastener member(s) will result in one or both of the fastener member thread helixes being damaged.
The purpose of the lead-in is to reduce axial misalignment. Therefore, when a shorter lead-in is used, the diameter of the lead-in generally needs to be made larger so as to properly restrict axial misalignment, see FIG. 3. A larger lead-in diameter, however, makes it more difficult to feed the lead-in into the internal thread, and thus the less effective is the desired axial alignment.
What is needed is a short lead-in located at one end of an externally threaded fastener member that will not jam when inserted into an internally threaded fastener member at typical misalignment angles during an assembly process.